1. Field of the Inventions
The aspects and advantages of the present inventions generally relate to apparatus and methods of photovoltaic or solar module design and fabrication and, more particularly, to packaging techniques for solar modules such as solar modules employing thin film solar cells.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon, which is in the form of single or polycrystalline wafers. However, the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by the more traditional methods. Therefore, since early 1970's there has been an effort to reduce cost of solar cells for terrestrial use. One way of reducing the cost of solar cells is to develop low-cost thin film growth techniques that can deposit solar-cell-quality absorber materials on large area substrates and to fabricate these devices using high-throughput, low-cost methods.
Group IBIIIAVIA compound semiconductors comprising some of the Group IB (Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se, Te, Po) materials or elements of the periodic table are excellent absorber materials for thin film solar cell structures. Especially, compounds of Cu, In, Ga, Se and S which are generally referred to as CIGS(S), or Cu(In,Ga)(S,Se)2 or CuIn1-xGax (SySe1-y)k, where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employed in solar cell structures that yielded conversion efficiencies approaching 20%. Absorbers containing Group IIIA element Al and/or Group VIA element Te also showed promise. Therefore, in summary, compounds containing: i) Cu from Group IB, ii) at least one of In, Ga, and Al from Group IIIA, and iii) at least one of S, Se, and Te from Group VIA, are of great interest for solar cell applications. It should be noted that although the chemical formula for CIGS(S) is often written as Cu(In,Ga)(S,Se)2, a more accurate formula for the compound is Cu(In,Ga)(S,Se)k, where k is typically close to 2 but may not be exactly 2. For simplicity we will continue to use the value of k as 2. It should be further noted that the notation “Cu(X,Y)” in the chemical formula means all chemical compositions of X and Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example, Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly, Cu(In,Ga)(S,Se)2 means the whole family of compounds with Ga/(Ga+In) molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from 0 to 1.
The structure of a conventional Group IBIIIAVIA compound photovoltaic cell such as a Cu(In,Ga,Al)(S,Se,Te)2 thin film solar cell is shown in FIG. 1. A photovoltaic cell 10 is fabricated on a substrate 11, such as a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. An absorber film 12, which includes a material in the family of Cu(In,Ga,Al)(S,Se,Te)2, is grown over a conductive layer 13 or contact layer, which is previously deposited on the substrate 11 and which acts as the electrical contact to the device. The substrate 11 and the conductive layer 13 form a base 20 on which the absorber film 12 is formed. Various conductive layers comprising Mo, Ta, W, Ti, and their nitrides have been used in the solar cell structure of FIG. 1. If the substrate itself is a properly selected conductive material, it is possible not to use the conductive layer 13, since the substrate 11 may then be used as the ohmic contact to the device. After the absorber film 12 is grown, a transparent layer 14 such as a CdS, ZnO, CdS/ZnO or CdS/ZnO/ITO stack is formed on the absorber film 12. Radiation 15 enters the device through the transparent layer 14. Metallic grids (not shown) may also be deposited over the transparent layer 14 to reduce the effective series resistance of the device. The preferred electrical type of the absorber film 12 is p-type, and the preferred electrical type of the transparent layer 14 is n-type. However, an n-type absorber and a p-type window layer can also be utilized. The preferred device structure of FIG. 1 is called a “substrate-type” structure. A “superstrate-type” structure can also be constructed by depositing a transparent conductive layer on a transparent superstrate such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga,Al)(S,Se,Te)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In this superstrate structure light enters the device from the transparent superstrate side.
There are two different approaches for manufacturing PV modules. In one approach that is applicable to thin film CdTe, amorphous Si and CIGS technologies, the solar cells are deposited or formed on an insulating substrate such as glass that also serves as a front protective sheet or a back protective sheet. In this case the solar cells are electrically interconnected as they are deposited on the substrate. In other words, the solar cells are monolithically integrated on the substrate as they are formed. These modules are monolithically integrated structures. For CdTe thin film technology the substrate is glass which also is the front protective sheet for the monolithically integrated module. In CIGS technology the substrate is glass or polyimide and serves as the back protective sheet for the monolithically integrated module. In monolithically integrated module structures, after the formation of solar cells which are already integrated and interconnected in series on the substrate, an encapsulant is placed over the integrated module structure and a protective sheet is attached to the encapsulant. An edge seal may also be formed along the edge of the module to prevent water vapor or liquid transmission through the edge into the monolithically integrated module structure.
In standard Si module technologies and for CIGS and amorphous Si cells that are fabricated on conductive substrates such as aluminum or stainless steel foils the solar cells are not deposited or formed on the protective sheet. They are separately manufactured and then the, manufactured solar cells are electrically interconnected by stringing them or shingling them to form solar cell strings. In shingling, individual cells are placed in a staggered manner so that a bottom surface of one cell makes direct physical and electrical contact to a top surface of an adjacent cell. Therefore, there is no gap between two shingled cells. Stringing is typically done by placing the cells side by side with a small gap between them and using conductive wires or ribbons that connect an electrical terminal of one cell to an electrical terminal of an adjacent cell. Strings obtained by stringing or shingling are then interconnected to form circuits. Circuits may then be packaged in protective packages to form modules. Each module typically includes a plurality of strings of solar cells which are electrically connected to one another. The solar modules are constructed using various packaging materials to mechanically support and protect the solar cells in them against mechanical damage. The most common packaging technology involves lamination of circuits in transparent encapsulants. In a lamination process, in general, the electrically interconnected solar cells are covered with a transparent and flexible encapsulant layer which fills any hollow space among the cells and tightly seals them into a module structure, preferably covering both of their surfaces. A variety of materials are used as encapsulants, for packaging solar cell modules, such as ethylene vinyl acetate copolymer (EVA) and thermoplastic polyurethanes (TPU). However, in general, such encapsulant materials are moisture permeable; therefore, they must be further sealed from the environment by a protective shell, which forms a barrier to moisture transmission into the module package. The protective shell generally includes a front protective sheet, a back protective sheet and an edge sealant that is at the periphery of the module structure (see for example, published application WO/2003/050891, “Sealed Thin Film PV Modules”). The top protective sheet is typically glass, but may also be a transparent flexible polymer film such as TEFZEL® (a product of DuPont), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and the like. The top polymeric film may have a moisture barrier coating on it. The back protective sheet may be a sheet of glass or a polymeric sheet such as TEDLAR® (a product of DuPont). The back protective polymeric sheet may also have a moisture barrier layer in its structure such as a metallic film like an aluminum film. Light enters the module through the front protective sheet. The edge sealant is a moisture barrier material that may be in the form of a viscous fluid which may be dispensed from a nozzle to the peripheral edge of the module structure or it may be in the form of a tape which may be applied to the peripheral edge of the module structure. There are a variety of such edge sealants provided to solar module manufacturers. It should be appreciated that in the above described non-monolithic module structure where separate pieces of solar cells are interconnected and then encapsulated on both surfaces by an encapsulant, the encapsulant becomes a conduit through which moisture may travel to all regions of the solar cell, front and back.
FIG. 2A shows a prior art solar module 50 including a first string 52A of solar cells and a second string 52B of solar cells. The first string 52A includes solar cells A1, A2 and A3, the second string 52B includes solar cells B1, B2 and B3. The solar cells in each string are electrically interconnected with one another. The strings 52A and 52B are also electrically connected with one another. The interconnections between cells and strings are not shown in the figure to simplify the drawing. As shown in FIG. 2B in cross section, the solar cells are encapsulated by encapsulant material 54 and sandwiched between a top or front protective sheet 56, typically glass, through which the light enters and a back protective sheet 58, and a bottom or back protective sheet 58, which may be glass or a polymeric sheet. An edge sealant 60 seals the edges of the protective sheets. The protective sheets and the edge sealant 60 form a protective shell of the solar module 50, which protects the solar cells encapsulated by the encapsulant material from outside conditions such as moisture. Although the exemplary prior art solar module design shown in FIG. 2A has six solar cells, many more electrically connected solar cells can be packed into the protective shell. However, one disadvantage associated with this design is the fact that any defect in the protective shell that causes moisture to get inside the module structure causes complete failure of the whole module. Once moisture gets into the protective shell it diffuses fast through the encapsulant material which is a poor moisture barrier. Such moisture diffusion through substantially the whole inside volume of the protective shell results in corrosion and malfunction of the entire solar cell population within the protective shell. Defects in the protective shell may occur in the edge sealant or in the front or back protective sheets. For the newly developed flexible module structures such as flexible modules employing flexible CIGS or amorphous Si solar cells fabricated on metal foil substrates, this concern of defectivity is even more important compared to the module structures employing glass protective sheets. Since the flexible module structures employ thin polymeric materials as the front and back protective sheets, preferably with moisture barrier coatings or layers, any defects in the polymeric sheets and/or the moisture barrier coatings or layers would cause moisture to enter the module structure through the front or back protective sheets and cause failure as described above. Since the total area of the front and back protective sheets is much larger than the cross sectional area of the edge sealant through which moisture may enter, the probability of defect formation in the large area front and back protective sheets is high in flexible and large module structures.
From the foregoing, there is a need in the solar cell manufacturing industry, especially in thin film photovoltaics, for better packaging techniques that can provide reliable performance at reduced cost. For example, CIGS solar cells are being developed for their low cost and high efficiency. However, the long term reliability of CIGS modules depends on the ability of the module package to keep the moisture away from the solar cells for over 20 years. It should be noted that CIGS solar cells are sensitive to moisture and they need to be protected, especially in non-monolithic module structures where individual CIGS cells are interconnected and then encapsulated in an encapsulant.